This invention relates to turbine meters of the type used to measure the flow of fluids by converting kinetic energy of a flowing fluid to rotation of a turbine, and more specifically, to rotors and rotor blades for such turbine meters.
Turbine meters are used to measure the flow of fluids by converting kinetic energy of the flowing fluid to rotation of a turbine. While turbine meters can measure both the flow of liquids and the flow of gases, the theory of operation of gas turbine meters differs somewhat from that of liquid driven meters due to the differences in the density and kinematic viscosity of the two fluids.
Since liquids are essentially incompressible, the density of liquids does not vary significantly with pressure or temperature. Also, the density of liquids is relatively high so there is ample driving torque from liquid flow to overcome mechanical friction in the meter. Thus, small changes in retarding torques, for example due to increases in friction between moving parts, do not affect the performance of liquid turbine meters. Conversely, the density of gas is relatively low so that gas turbine meters are highly sensitive to changes in retarding torques within the meters, especially at low pressure and low flow rates. Changes in kinematic viscosity, however, do affect the performance of both gas turbine meters and liquid turbine meters.
Referring to a gas meter by way of illustration, the total volume of gas passing through the meter is determined by counting the number of revolutions of a measuring rotor mounted within the meter. Gas turbine meters are known as inferential meters because they infer how much gas has passed through by observing something other than the displacement of gas; i.e. gas turbine meters infer how much has passed by measuring the speed of the rotor rotation. A gas turbine meter is a gas velocity measuring device. The actual flow rate can be inferred from the velocity of the gas because the cross-sectional area of the annular passage preceding the rotor is a known quantity.
The driving energy to turn the rotor is the kinetic energy, or energy of motion, of the gas being measured. The gas impinges on rotor blades mounted on the measuring rotor and overcomes retarding forces that inhibit the rotor from turning.
A conventional gas turbine meter typically includes an elongated, cylindrical housing which forms a flow path for gas which is flowing within a pipeline in which the housing is mounted. An inlet flow straightener is mounted adjacent an inlet port in the housing to cause gas flowing from the inlet port to flow in an axial direction within the housing. A measuring rotor is mounted within the housing downstream of the inlet flow straightener so as to rotate about its axis of rotation. In an axial turbine meter the axis of rotation is also the central axis of the cylindrical housing. The measuring rotor has an upstream end and a downstream end with respect to the flow of gas through the housing.
The measuring rotor has turbine blades mounted on it at an angle with respect to its axis of rotation to cause the rotor to rotate at a speed approximately proportional to the velocity of the gas flowing through the housing. Each of the rotor blades has a high pressure surface which faces toward the flow of gas within the housing and a low pressure surface which faces away from the flow of the gas. Each turbine blade also has a leading edge at the upstream end of the rotor and a trailing edge at the downstream end of the rotor.
Gas turbine meters have typically been constructed with a metal cylindrical housing having a removable measuring cartridge mounted within it. The measuring cartridge normally includes at least the measuring rotor, its rotor bearings and a coupling for interconnecting the measuring rotor to a mechanical register mounted on top of the measuring cartridge. The rotor blades are usually mounted on a rotor cylinder, which forms the hub of the rotor.
Gas Meter Accuracy
Gas turbine meters are commonly installed in pipelines used in the natural gas industry for the measurement of the flow of large volumes of gas. The volumes are often large so that small errors in measurement can result in large losses of revenue to gas transmission companies and local distribution companies. An example of the magnitude of losses which can occur was presented in a 1992 technical publication of the Netherlands Measurement Institute. Consider a 12-inch turbine meter operating at a pressure of 580 psig and having a gas volume which is 59% of maximum capacity. Assuming the cost of natural gas is $0.0037 per cubic foot, an error of only 0.2% results in a loss of revenue of $160,000 per year. Clearly it is vital to maintain the accuracy of gas turbine meters.
Each gas turbine meter must be separately calibrated to determine its accuracy after it is manufactured. Calibration is necessary because normal, minor variations in meter components cause each gas turbine meter to register a slightly different volumetric flow for a given volume of gas. By way of example, from meter-to-meter blades on otherwise identical turbine measuring rotors vary slightly in shape due to minor manufacturing inconsistencies. As a result, each turbine measuring rotor rotates at a slightly different speed for gas flowing at the same velocity. Similarly, separate sets of measuring rotor bearings of the same make and model can impose slightly different frictional forces on the rotors of separate meters on which they are mounted. Additionally, the gas turbine meter's mechanical register, sometimes called an index, gives a reading of gas flow volume on a set of dials. The register is typically connected to the turbine measuring rotor through a coupling which includes gears, magnetic couplings and other components which load the turbine rotors of different gas turbine meters to a somewhat different extent. As a result, each gas turbine meter will register its own unique flow level for a given volume of gas.
At the time of manufacture of a gas turbine meter, the accuracy of a meter is proved by testing the meter against a known standard such as a master meter or a bell prover or a sonic nozzle. At a given temperature, a given gas line pressure and a given gas flow rate, the volume of gas registered by the meter is compared to the actual volume of gas which flowed through the meter as determined by the known standard. This ratio of the volume of gas measured by a meter's mechanical register to the actual volume of gas flowing through the meter is called the accuracy of the meter. The calibration factor of a meter, referred to by the letter "K," expressed in terms of pulses per unit of volume flowing through a meter. The calibration factor "K", is the amount by which the registered reading of the meter is divided to get a 100% accurate volume reading. For each of a given series of line pressures at which a gas turbine meter may operate, the K factors are determined for a range of flow rates expected for the meter. A table of these K factors is normally provided with each meter.
As a result of the calibration of the meter, the accuracy of the meter at a given line pressure can be graphed over a range of gas flow rates. See FIG. 6 by way of example which shows a graph of two accuracy curves. The lower of the two curves 10 is a typical accuracy curve of a gas turbine meter having rotor blades which do not incorporate this invention. The upper curve 11, which will be discussed more fully in the Description of the Preferred Embodiments, is an accuracy curve of a gas turbine meter using this invention. Along the Y-axis of FIG. 6 is a measurement of the "percent accuracy" of the meter. Along the X-axis is a measurement of the "flow rate" of the meter in terms of the percentage of the total flow rate capacity of the meter. The resulting graphed curved line is the accuracy curve.
The meaning of the graph is as follows: If the amount of gas measured by the meter is equal to the actual amount of gas that has passed, the meter accuracy is 100%. Thus, if the meter reads 100 units of gas and 100 units have actually passed, the meter would be 100% accurate. If the amount of gas measured by the meter is less than the amount of gas that actually flowed, the meter percentage accuracy is said to be less than 100%, and there is a false low. For instance, if a meter reads 99 units of gas when in actuality 100 units of gas has flowed, the meter would be 99% accurate. The meter would be undermeasuring the amount of gas that has actually flowed and the customer will pay too little. If the amount of gas measured is more than the amount that has actually flowed, the meter's percentage accuracy is said to be over 100%, and there is a false high. For instance, if a meter reads 102 units of gas when only 100 units actually flowed, the percent accuracy would be 102%. The meter is overmeasuring the amount of gas that has actually flowed and the customer will pay too much.
As indicated above, the accuracy of the volume of gas recorded on the dials of a meter's register is checked at the time of a meter's calibration over a range of the meter's operating conditions. Components of the meter, such as the gears and magnetic couplings between the measuring rotor and the register, are often modified to attempt to get the accuracy of the meter as consistent as possible over its expected range of gas flow rates.
If the gas meters were inaccurate by the same amount across all flow rates, fewer problems would exist. If a meter consistently overmeasured or undermeasured by the same amount all the time, a correction could easily be made by using the gears or other correcting mechanisms to correct the readings of the register. The correcting mechanism would simply shift the accuracy curve up or down so that it would be 100% accurate all of the time.
However, as can be seen by examining the lower curve 10 of FIG. 6, gas turbine meters do not have the same percentage of error across all flow rates. The accuracy curve 10 is an accuracy curve of a typical prior art gas turbine meter. This gas turbine meter is equipped with 8 inch helical rotor blades which have a mean helical angle of 45.degree. through the axis of rotation of the rotor on which the blades are mounted. The accuracy curve 10 is not linear in that it does not have a constant percentage of error across all flow rates of the meter at the temperature and pressure at which the meter was tested. A linear accuracy curve, which is a desirable characteristic in gas meters, would be a straight line. The accuracy curve 10 shows a non-linear distribution of the percentage of accuracy for a gas turbine meter which is generally representative of most prior art gas turbine meters. The accuracy curves of these turbine meters tend to have an undesirable "hump" at low flow rates at between 10-20% of the maximum flow rate of a meter. Thus, the accuracy curve 10 has a high reading of approximately 102% at point 12 and the readings above and below this flow rate decrease appreciably. Readings below about 5% of the meter capacity measured at atmospheric pressure, become unreliable because gas tends to slip past the clearance between the rotor blades and the walls of the housing in which the rotor is installed.
The accuracy measurement of the turbine meter at a particular line pressure, represented by the accuracy curve 10 in FIG. 6, tends to fall fairly rapidly as the flow rate of the meter increases. At point 14 on the accuracy curve 10, which is about 28% flow rate capacity of the meter, the percentage of accuracy of the meter has decreased to about 100.7%; at point 16, approximately 50% of the flow rate capacity, the percent of accuracy of the meter is 100%; while at point 18, approximately 100% flow rate capacity of the meter, the percent accuracy is about 99.3%. Between about 5% and 100% of the flow rate capacity of this meter the accuracy ranges from a high of 102% to a low of about 99.3%. Thus, the linearity of this meter, that is the difference between the highest percent of accuracy of the meter and its lowest percent of accuracy over the meter's operating range, is about 2.7%.
Only a single set of gears and/or couplings can be installed at one time between the measuring rotor and the dials of a meter's register. Thus, the register can only be calibrated to be 100% accurate at one flow rate, called the change gear rate, which is usually about 50% or 60% of the maximum flow rate of the meter. At other flow rates significant inaccuracies must sometimes be tolerated. As shown by accuracy curve 10, because gas meter accuracy curves are not linear, at some flow rates turbine meters typically undermeasure the amount of gas that has flowed, while at other flow rates they tend to overmeasure the amount of gas that has flowed. The non-linearity of the accuracies of these meters over the range of expected flow rates is difficult to compensate for while calibrating the meter and can result in an undesirable range of inaccuracies for the meter.